In many very large scale integration (VLSI) semiconductor devices, polycrystalline silicon (polysilicon) gates are formed over a gate dielectric layer. Due to the relatively high resistance values of the polysilicon film, a tungsten silicide film is often formed to lower resistance along the polysilicon gate. Generally, the tungsten silicide film is formed using tungsten hexafluoride (WF.sub.6) and silane SiH.sub.4). The fluorine from the tungsten hexafluoride can passivate interface traps, which is good. However, the fluorine can also break the silicon-oxygen bond at the silicon-gate oxide interface, which is undesired because it creates more silicon dangling bonds, increases the gate dielectric layer thickness, and decreases the dielectric constant of the gate dielectric layer.
One attempt to reduce the problem with the fluorine is to increase the ratio of silane to tungsten hexafluoride during the tungsten silicide formation. Typically, the tungsten silicide film is formed with a ratio of silicon to tungsten of approximately 2.1:1 to 2.6:1. However, the increased silicon content increases the resistivity of the tungsten silicide film.
Another attempt to reduce the fluorine-related problems is to form sputtered tungsten silicide film by sputter deposition. However, this is done at the expense of increased particle problems. Similarly, a tungsten nitride film can be placed between the polysilicon and the tungsten silicide, but the tungsten nitride is typically formed by sputter deposition. Again, sputtering usually has more particle problems than chemical vapor depositions.
A need exists to form a gate electrode that does not cause gate dielectric thickening or other changes that affect the electrical performance of a semiconductor device. A need further exists to form the gate electrode without having to develop complex processing steps.